: Measurement error in risk factors is a commonly identified problem in cancer and nutritional research. Many important examples can be found in epidemiological investigations of the cancer prevention/ promotion properties of dietary nutrients. Uncertainties in the reliability of questionnaire data, the relevant time frame of observation, and the nutrient composition of food can obscure and bias the effects observed in conventional studies. Measurement error methods for epidemiological regression models attempt to deal with this issue by incorporating information on the magnitude of uncertainties in exposure estimates into procedures for designing and analyzing epidemiological studies. New methods are proposed for a number of practical and technical issues related to the measurement error problem and will be applied to important ongoing studies in cancer epidemiology. The specific aims fall into three broad areas: methods for exposure measurement error in nutritional epidemiology, design issues for studies with measurement error, and issues of special importance in longitudinal studies. In nutritional epidemiology, a comprehensive approach will be developed for assessing the relative impact of all major sources of errors in the assessment of nutrient intakes. Ongoing research on special methods for ordinal categorizations of nutrient intake will be extended to include simultaneous assessment of several nutrients. In addition, further research on estimating the reliability of noisy dietary measures for designs where: (a) an unequal number of replicates per subject are available or (b) normality of the dietary measures are not assumed will be performed. The design issues feature further development and application of multi-stage adaptive designs involving resampling for validation studies. Finally, new longitudinal methods are proposed for dealing with measurement error in a risk factor expressed as a proportion, such as the proportion of follow-up surveys reporting aspirin use, and for estimation of the variance parameters of random effects in a longitudinal validation study of dietary intakes and nutrient concentrations in serum.